In multi-carrier communication systems, orthogonal frequency division multiplexing (OFDM) is an effective, high-speed communications technique that allows for relatively efficient multi-path channel equalization. However, signals generated using traditional OFDM techniques tend to suffer from relatively large peak-to-average ratios (PARs) or peak-to-average power ratios (PAPRs), which in turn may lead to significant distortion noise and low power efficiency in peak-limited channels. In addition, under relatively harsh channel conditions, transmitted OFDM signals tend to incur significant timing offsets and carrier frequency offsets. Because traditional OFDM techniques tend not to be robust under harsh channel conditions, significant timing offsets may result in inter-block interference, and significant carrier frequency offsets may result in inter-carrier interference. Both of these forms of interference are detrimental to the bit error rates of received signals.
In order to mitigate these detrimental effects, some traditional OFDM methods include, on the transmitter side, transmitting a synchronization and/or channel estimation preamble in conjunction with and preceding each transmit information sequence. On the receiver side, the preamble is used during signal acquisition to synchronize to the received signal and, when the preamble also includes channel training information, it also may be used to perform channel estimation (e.g., estimating transmission channel parameters such as timing offset, carrier frequency offset, and multi-path fading). Although transmission of a preamble is relatively simple to implement, a tradeoff to implementing this technique is that a significant amount of bandwidth is used solely for preamble transmission, and thus for synchronization, acquisition, and, when channel training information is available, also for channel estimation. Furthermore, using preamble-based techniques, there are no options for re-acquiring a signal during the information sequence reception portion of the transmission if synchronization is lost after the preamble is received.
Another traditional OFDM method excludes the use of a preamble for synchronization, and instead a cyclic prefix (or cyclic extension) is included within each transmitted OFDM symbol. Using a cyclic prefix, the first G samples represented in the OFDM symbol are an exact copy of the last G samples of the inverse fast Fourier transform (IFFT) baseband output. The cyclic prefix can be used for timing and frequency synchronization, but this method is generally less robust than using methods that include transmission of a preamble because G typically is selected to be less than the length of a typical preamble (e.g., a preamble having length L). Comparatively, for example, if L≈10G, the detection performance (e.g., the detection signal-to-noise ratio (SNR)) should be about 10 dB higher using a preamble of length L compared to a cyclic prefix of length G. Because each OFDM symbol contains a cyclic prefix, a correlation may be performed on each OFDM symbol, and then the correlations may be integrated over multiple symbols (e.g., 10 OFDM symbols) in order to achieve substantially equivalent detection SNR. However, this incurs a processing delay proportional to the number of OFDM symbols over which the correlations are integrated (e.g., a processing delay of 10 OFDM symbols), which is unacceptable in many applications. In addition, a cyclic prefix is not useful for PAR reduction.
In order to mitigate the effects of the channel on a received signal, some traditional OFDM methods also perform channel estimation, on the receiver side. The channel estimate may be used to compensate for clipping associated with a limited channel dynamic range, timing offsets, carrier frequency offsets, and multi-path fading among other things. Naturally, the channel estimate has some error, when compared with actual channel conditions. Traditional OFDM transmission methods may experience an increase in channel estimation errors on the receiver side, which may result from non-linear amplification, by a power amplifier device on the transmitter side, of transmit information sequences having higher than desired peak-to-average power ratios. Such non-linear transmission may cause significant out-of-band interference (i.e., interference outside the signal bandwidth, such as in the adjacent channels and/or other user channels), and also may induce undesired in-band interference, which adds distortion the transmitted information bits and also to the channel training information. Furthermore, improper synthesis of the channel training information may lead to further channel estimation errors at the receiver. Thus, non-linear amplification of high peak-to-average power ratio signals and improper channel training information design may, in the receiver, result in unacceptably high channel estimation errors and excessively high bit error rates.
Accordingly, what are needed are methods and apparatus for communicating wireless signals in multi-path communication systems, in which synchronization and acquisition are performed in a bandwidth-efficient manner that is relatively robust under harsh channel conditions. Also needed are methods and apparatus that enable re-acquisition of a signal to be performed if synchronization is lost during the information sequence reception portion of a transmission. Also needed are methods and apparatus for communicating wireless signals having improved peak-to-average power ratios and/or bit error rates, when compared with traditional methods and apparatus. Other features and characteristics of the inventive subject matter will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.